Humidity sensitive device



Sept. 9, 1952 S. LARACH HUMIDITY SENSITIVE DEVICE Filed NOV. 30, 1949 Patented Sept. 9, 1952 HUMIDITY SENSITIVE DEVICE Simon Larach, New Brunswick, N. .1., assignor to Radio Corporation of America, a corporation of'Delaware Application November 30, 1949, Serial No. 130,120

This invention relates tor humidity-sensitive devices and to methods of preparing these devices. More `particularly, theV invention relates to improvements in that type of` device utilizing a .conductive nlm-ofmaterial, -the electrical resistance of which varies with its moisture content, which, in turn, is dependent upon the relative humidity of theY sur-rounding atmosphere.

Several different types of humidity measuring devices have been in use in the past. One or" these is the psychrometer, which makes use of a thermodynamic method of measurement. An ekample of this` type of device is the commonly used Wet and dry bulb thermometer. Difference in. evaporation lrate from the Wet bulb occurs las the relative humidity of the surrounding atmosphere varies, an'dthis, Ain'ltur'n, causes the .temperature reading of the'ivet bulb thermometer to vary. Another is a device v`making use of a hgroscopic substance such as a stretched lanient ofhair'. In thistype, the filament stretches and. contracts with variations in moisture content' and a. mechanical coupling 'movement amplii'es the change in length and transmits the change. to a pointer'movingfover a dial. Another type relies onA moisturercondensation on a relatively cool surface. An example of this is the dew-point hygrometer Yfor nding the dew-point ofthe surrounding atmosphere in relation to the temperature. All of. these three types have seriousl detects. They are open to gross errors inz'indica'tion and usually have a rather long time lag'before equilibrium is reached and a reading can be obtained. Vlfith the exception of the hair hygrometer, they,vr also, cannot be readily used to givecontinuous readings of the humidity.

Still another type, of device. makes use of an absorption method.' VA ysubstance is used which readily absorbs atmospheric moisture at a rate dependentl upon relative humidity. The change in moisture content after a certain xed time interval is then determined either gravimetrically or'vo'lumetrlcally, 'depending on the substance. used. Although reasonably accurate, use of this type of device is limited' since it cannot give continuous readings and considerable time and effortjare needed* to make a single determination.

Finally, another type of device, which has been previously proposed, makes use of a iilm of hygroscopic material, the film being electrically conductive and having a resistance Which varies with vthe moisture content of the lm. It is to improvements in this latter type of device that the present invention is directed. This type of device usually has little or no time-lag in responsefand is continuous reading, making it ap- 2 Y, Y. plicable forr situations where continuous sampling of the humidity is desired such as in radiosonde.

Former humidity-sensitive devices of the electrical resistance type have utilized various hygroscopic substances for the sensitive element..Y

These have included saturated solutions of lithium chloride, calcium chloride, and zinc chloride, and various hydrophilic gels such as those of gelatin, pectin,` and agar-agar. Al-

though these generally have short time lag in,

ploying a different humidity-sensitive resistance nlm material.V The nlm is formed by covering an electrically insulating plate with a thin layer of a solution of ar tetrachloride of a metal from the class consisting of tin, zirconium, hafnium, and

lead. The solution is then heated at about C. until it partially dries to become tacky.v

Electrodes are then provided in electrical contact with the tacky lm and a low D.. C. voltage is applied across it. A sensitive current meter can then be used to indicate changes in resistivity ofthe film. This current meter can be calibrated to read directly inpercent relative humidity.

One object of the present invention is to provide improved materials having an electrical resistance which varies with. changes in relative humidity of the surrounding atmosphere.

Another object is to provide novel methodsA of preparing lms of improved materials, the resist.- ance of which is humidity-sensitive.

YAnother object is to provide a humidity-sensitive ilm of a material which does not exhibit D. C. polarization.

A further object is to provide improved humidity-sensitive elements for use in a humidityimproved humidity-measuring apparatus adapted to give continuous readings of humidity conditions of the surrounding atmosphere.

Still another object of the invention is to provide improved humidity-measuring apparatus of relatively simple construction and operation.

These and other'l objects will be more apparent and the invention will be more readily understood from the following detail description, including the drawings of which,

Figure 1 is a cross-section View of one modification of a humidity-sensitive element prepared in accordance with the present invention,

Figure 2 is a perspective view of a particular modification of a device including a humiditysensitive element such as illustrated in Figure 1,

Figure 3 is a schematic diagram of a humidity measuring apparatus including a preferred modification of a device as illustrated in Figure 2,

Figure 4 is a graph of relative sensitivity to changes in humidity versusmolarity of solution used in preparing a film such as shown in Figure 1, and

Figure 5 is a calibration curve of current meter reading versus relative humidity for a device such as illustrated in Figures 2 or 3.

"Ihere will now be described a preferred eX- ample of preparation of a humidity sensitive film'in accordance with the present invention, referring to Figure 1. First a 1/5 moiar aqueous solution of stannic chloride, SnCli, is prepared. A small volume of this solution; for example, 1 or 2 drops, is placed on a clean surface of an electrical insulating plate 2, such as a glass microscope slide. The insulating plate may be anysuitable material having a non-porous, preferably smooth, polished surface since the plate acts only as a convenient base for supporting the lm of solution. The base material should also be fairly heat resistant. The surface containing the film of solution is then heated for at least about minutes and at a temperature between'about 80 C. and about'lOO" C. until the film becomes tacky. In this condition the film 4 has lost most of the water present before heating took place but some still remains. Exactly in what form the water is combined with the stannic chloride is not known. It may be present aswater of hydration or simply be mechanically held. It may also be that some of the stannicchloride has been converted to stannic oxychloride and that some hydrochloric acid has been formed. In any event, some of the water is still present in some condition.

Once the film has been heated so that it becomes tacky, furtherheating'within the temperature range specified produces no change in results. Various heating times of up to `onehour have been used with the lm remaining in the same condition it assumed after the first 15 minutes -of heating. It appears that, generally speaking, the heating time is not critical except' that the lm should be heated long enough so that it assumes a condition of tackiness.

'Temperature of heating,liowever, is critical. If heating occurs below about 80 C., the film is not converted to a tacky state using heating periods of reasonable length and, if heated much above about 100 C., the condition of the film also sults are quite uniform, which indicates that the material has assumed some definite physical or chemical state when it becomes tacky. In this state, the lm is in a state of extremely delicate equilibrium with the surrounding atmosphere. It has been found that relatively small changes in` humidity will quickly change the electrical resistivity of the nlm. l

The property of change of resistivity of the film with changes in the humidity of the surrounding atmosphere can be utilized by making a device, a simple embodiment of which is illustrated in Figure 2. In the device, illustrated in perspective in this figure, the glass base 2 has been provided with a pair of electrodes 6 and 8 positioned adjacent opposite edges of a surface of the base body. These electrodes may be stripes of silver paste applied with a brush or they may be silver paste electrodes red-on in the manner customary with applying such electrodes to ceramic capacitors. Next, there is applied over the surface of both the base 2 and the electrodes a film of stannic chloride. The latter film is app-lied exactly as described above.

There has thus been described a simple device which may be utilized as the essential element in various forms of apparatus for making continuous measurements of the relative humidity of the atmosphere at a particular location.

A particular form of humidity-measuring apparatus utilizing the humidity-sensitive element of the present invention will now be described with reference to Figure 3, there may be used a humidity-sensitive element, such as shown in Figure 2 comprisingv a glass base 2, but having a pair of electrodes I0 and I2 of more efficient design. These electrodes may each comprise a series of interlaced ngers` I4 connected to a common lead i6 orl 8, both fingers and leads being composed of stripes of silver paste applied to the glass base with a iine carnels hair brush. Av tacky nlm of stannic chloride `is applied over the electrode structure, as pre.- viously described. Each 'of the electrodes l0 and I2 is connected to an opposite poleof a low volt-v age source of D. C. 20 such as a battery, and amicroammeter 22 is connected in series with the voltage source and the electrodes lil and l2. `As the resistance of the lm 4 varies with changes in the relative humidity, correspondingchanges will occur in the current reading observed on the As the relative humidity rises,

inicroammeter. the resistance o1" the film decreases and the current reading on the microammeter increases.

Once the calibration curve is known, readings on the microainmeter can quickly be converted tol percent relative humidity. The calibration. curve can be obtained with the aid of anotherY type of humidity-measuring device such as psychrometer.

' An apparatus such as above described may be made a part of an automatic, continuous-re- In this apparatus,

, A typical calibration curve is shown in Figure 5."

tacky film to changes in relative humidity plotted against concentration of the solution in mals/liter. As shown in the graph, solutions of from 0.1 to 1 molar may be used, although films prepared from a 1/5 molar solution have been found most sensitive and this concentration is therefore preferred.

Although the tetrachloride of tin is preferred as the salt from which to prepare the sensitive film, there may also be used the tetrachlorides of any one of zirconium, hafnium, or lead. Concentrations of solution and times and temperatures of heating are similar to those found useful in the case of the tin salt.

The present invention is not to be confused with previous inventions relating to conductive films of stannic chloride. It has previously been proposed, for example, to apply vapors of stannic chloride to a surface of a ceramic body heated to redness. In this case, a clear, hard conducting film is formed, which has suiilcient electrical resistance to enable it to be heated by application of a current. This type of film is not tacky and is resistant to abrasion but, on the other hand, is not known to be humiditysensitive.

There has thus been described an improved device for measuring relative humidity and which is applicable to apparatus for giving continuous, automatic readings. In its preferred form, the apparatus has almost no time lag and has high sensitivity to slight changes in the relative humidity of the atmosphere. The device has the further advantage ofsimplicity of construction and operation.

I claim as my invention:

1. A method of preparing a humidity-sensitive element comprising covering a surface of a non-porous, electrically insulating body with a thin film of an aqueous solution of a salt from the class consisting ofthe tetrachlorides of tin, zirconium, hafnium, and lead, and heating said surface to a temperature of about 80100 C. until said film is dried to tackiness.

2. A method according to claim 1 in which said salt is the tetrachloride of tin.

3. A method according to claim 2 in which said solution has a concentration of from about 0.1'molar to 1 molar.

4. A method of preparing a humidity-sensitive device comprising positioning a pair of electrodes on a surface of an electrically insulating body, covering said electrodes and said surface with a thin film of an aqueous solution of a salt from the class consisting of the tetrachlorides of tin, zirconium, hafnium, and lead and drying said iilm to tackiness at a temperature of from about 80 C. to about 100 C.

5. A method according to claim 4 in which said salt is the tin salt.

6. A humidity-sensitive element comprising a body of electrically insulating material having on a surface thereof a tacky film consisting essentially of a minor amount of Water and a salt from the class consisting of the tetrachlorides of tin, zirconium, hafnium, and lead.

7. A device according to claim 6 in which said salt is tin tetrachloride.

8. A humidity-sensitive device comprising a body of electrically insulating material having on a surface thereof a tacky film consisting essentially of a minor amount of water and a salt from the class consisting of the tetrachlorides of tin, zirconium, hafnium, and lead, said film having a pair of electrodes in contact therewith.

9. A device according to claim 8 in which said salt is tin tetrachloride.

10. In a humidity-indicating apparatus of the type comprising a body of electrically insulating material having on a surface thereof a film of a material of which the electrical conductivity varies with the humidity. a pair of electrodes in contact with said film, means for applying a D. C. potential across said film, and means for detecting changes in said conductivity, the improvement consisting in that said film is a tacky film consisting essentially of a minor amount of water and a salt from the class consisting of the tetrachlorides of tin. zirconium, hafnium, and lead.

11. Apparatus according to claim 10 in which said salt is tin tetrachloride.

12. In a humidity-indicating apparatus comprising a body of electrically insulating material having on a surface thereof a fihn of which the electrical conductivity varies with the humidity, means for applying a D. C. potential across said film, and means responsive to changes in conductivity of said film for indicating changes in the relative humidity of the atmosphere surrounding said fihn, the improvement consisting in that said film is a tacky film consisting essentially of water and a salt from the class consisting of the tetrachlorides of tin, zirconium, hafnium, and lead.

13. Apparatus according to claim 12 in which said salt is tin tetrachloride.

SIMON LARACH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,644,798 Tillyer et al. Oct. 11, 1927 2,377,426 Kersten June 5, 1945 2,381,299 McCulloch Aug. '7, 1945 2,458,348 Cleveland Jan. 4, 1949 

10. IN A HUMIDITY-INDICATING APPARATUS OF THE TYPE COMPRISING A BODY OF ELECTRICALLY INSULATING MATERIAL HAVING ON A SURFACE THEREOF A FILM OF A MATERIAL OF WHICH THE ELECTRICAL CONDUCTIVITY VARIES WITH THE HUMIDITY, A PAIR OF ELECTRODES IN CONTACT WITH SAID FILM, MEANS FOR APPLYING A D.C. POTENTIAL ACROSS SAID FILM, AND 